This invention relates to a method and apparatus for controlling hydrocarbon emission from glycol dehydrators in natural gas service.
Natural gas flowing from natural gas wells typically contains water vapor, condensible hydrocarbons and entrained liquids such as water droplets and oil droplets. It is common practice to condition the natural gas at a wellhead to remove these materials from the gas before it passes into the gas pipeline leading from the well. Such removal is necessary in order (1) to prevent clogging of pipelines with liquid water and liquid hydrocarbons which separate as a result of the lower temperatures existing in the transport system; (2) to prevent formation of hydrates which also can clog the system; and (3) to prevent corrosion of pipelines, tanks and other components of the system. The principles of such conditioning operations are well known. Water droplets are frequently removed by any of a variety of mechanical gas-liquid separators. Condensible hydrocarbons and additional water are generally removed by expanding the high pressure well gas so that the resulting temperature drop causes condensation of the condensibles, the condensed liquids being readily separated from the non-condensed gas. Subsequent dehydration of the gas is most commonly achieved by contacting it with a hygroscopic liquid, i.e. a liquid desiccant or absorbent such as diethylene or triethylene glycol, whereupon the water vapor is absorbed into the liquid desiccant. The desiccant is then regenerated by heating it to drive off water, and the resulting concentrated desiccant is recycled to the dehydration step. The heat required for regeneration is obtained by burning a portion of the natural gas.
Two of the essential pieces of equipment used for natural gas conditioning are a separator and a dehydrator, the former removing entrained liquids and condensibles and the latter removing water vapor. These are often separate pieces of equipment, but they may be combined in a single separator-dehydrator unit. The separator unit or separator section of a combined unit includes an expansion valve or choke for the natural gas and typically a stratification type water-hydrocarbon separator. The dehydrator unit is in principle a gas-liquid contact tower in which the wet gas is brought into intimate contact with strong glycol. The regenerator or reboiler is a separate unit usually in the form of a distillation column or the equivalent.
Examples of systems for conditioning raw natural gas at the well site to remove liquid components, condensibles and water vapor are described in a number of U.S. patents including U.S. Pat. Nos. 2,690,814; 2,812,827; 2,812,830; 3,212,238; 3,331,188; 3,367,089; 3,541,763 and 4,070,165.